To the Editors:

Concern regarding the deleterious bone effects of puberty suppression for transgender and gender diverse (trans) youth is an issue for patients, their families, and treating clinicians. Although there is considerable variability in access across jurisdictions, there has been an increase in demand for gender-affirming care for trans youth. Pubertal suppression with gonadotropin releasing hormone antagonist or agonist (GnRHa) therapy is typically commenced in early puberty (ie, Tanner stage 2) and aims to delay pubertal progression to allow maturation of the individual until an appropriate time for possible masculinizing or feminizing gender-affirming hormone therapy. Masculinizing hormone therapy typically involves testosterone therapy in standard doses used for hypogonadal men to achieve testosterone concentrations in the typical male reference range.

There are few studies in humans on the impact of GnRHa on bone. Retrospective and small uncontrolled cohort studies in trans youth have shown that even prior to GnRHa, bone mineral density (BMD) measured by dual-energy X-ray absorptiometry (DXA) may be lower than in age-matched youth.^(1-3) This is largely determined by factors such as lower body mass index, low vitamin D, suboptimal calcium intake, and lower physical activity.^(1-3) After GnRHa treatment, BMD declines, but there are also associated increases in body fat including in bone marrow adipose tissue.⁽⁴⁾ After gender-affirming hormone therapy, BMD Z-scores increase but may well remain below age-matched peers.^{(5, 6)} However, all existing studies have used DXA, which has poor precision and low correlation with fracture.⁽⁷⁾ Additionally, body fat increase with GnRHa influence photon attenuation which may artifactually underestimate BMD.⁽⁸⁾ Findings also vary depending on which reference range (male or female) is used and it is unknown whether timing of gender-affirming hormone therapy commencement matters.⁽⁹⁾ Additionally, it is difficult to extrapolate whether such changes impact peak bone mass accrual or whether observed changes are associated with a long-term risk of bone fragility or fracture in trans youth.

Although methodology to precisely measure the microarchitecture or breaking strength of bones in humans is challenging, this month's issue of the Journal of Bone and Mineral Research (JBMR) publishes a clinically relevant translational study which overcomes many of the limitations in existing literature.

Mimicking the clinical treatment regimens typically used for trans boys (assigned female at birth), Dubois and colleagues⁽¹⁰⁾ developed a mouse model to understand the impact of early puberty suppression on body composition, bone mass, and bone strength.

Prepubertal 4-week-old female mice were treated with degarelix as the GnRH antagonist which was followed by either placebo silastic implants, testosterone therapy from 6 weeks (early puberty), or testosterone therapy from 8 weeks (late puberty) onward. At the 16-week mark, considered early adulthood, outcomes were compared to untreated male and female mice.

Degarelix treatment significantly increased fat mass accumulation (restricted to white adipose tissue) and despite reduced food intake, fat mass was almost twofold higher than control animals of either sex.⁽¹⁰⁾ Degarelix also reduced lean mass and grip strength. Within 2 weeks of testosterone replacement (regardless of whether it was initiated early or late) body composition and bone marrow adiposity reversed and were similar to male controls. Grip strength was restored to female levels.

As expected, bone microarchitecture was sexually dimorphic with lower trabecular bone volume fraction (BV/TV) and cortical bone mass assessed by micro-computed tomography (μCT) at the femoral metaphysis in female compared with male mice. Degarelix reduced trabecular bone volume, cortical bone mass, and femoral bone breaking strength to levels lower than both male and female controls.⁽¹⁰⁾

Reassuringly, the negative effects of degarelix were counteracted by testosterone therapy. BV/TV increased, mostly due to an increase in trabecular number and decrease in trabecular separation, because effects on trabecular thickness were marginal.⁽¹⁰⁾ The duration of GnRHa therapy and the timing of testosterone administration was significant on the magnitude of increase in trabecular BV/TV. Testosterone replacement commenced in early puberty, increased BV/TV to the level of male controls by adulthood; however, if commenced in late puberty, BV/TV only increased to the level of female controls. Given that peak bone mass in mice is achieved at 4–6 months of age,⁽¹¹⁾ it remains unclear if a longer duration of testosterone therapy may further increase volumetric bone density. Cortical bone mass and femoral bone breaking strength was restored with testosterone up to female levels regardless of timing of testosterone replacement. Bone length was unaffected by GnRHa or testosterone therapy.

Although there are physiological differences between mice and humans, it is reasonable to hypothesize that for trans boys, fracture risk will not be increased after testosterone therapy provided that lifestyle factors are optimized for bone health (adequate vitamin D, physical activity, etc). Although the duration of GnRHa therapy is often difficult to control clinically (dependent upon when a patient presents for care, ease of access to GnRHa and readiness for hormonal therapy), earlier commencement of testosterone is more beneficial from a bone and body composition perspective. Certainly in adult trans men who have been on established testosterone therapy, it appears as though bone microarchitecture is preserved⁽⁸⁾ and no increased risk of fracture is seen when compared to cisgender individuals.⁽¹²⁾

This clinically relevant translational study is reassuring for trans boys who are treated with GnRHa followed by testosterone therapy. Further studies are needed in humans, particularly in trans girls (assigned male at birth) and trans women who may have compromised bone structure⁽⁸⁾ and further mouse models in trans girls are awaited.

致编辑们：

对于跨性别和性别多样化（跨性别）青少年的青春期抑制对骨骼的有害影响的担忧是患者、其家人和治疗临床医生面临的一个问题。尽管不同司法管辖区的获取机会存在很大差异，但跨性别青少年对性别肯定护理的需求有所增加。使用促性腺激素释放激素拮抗剂或激动剂 (GnRHa) 治疗的青春期抑制通常在青春期早期（即 Tanner 2 期）开始，旨在延迟青春期进展，以使个体成熟，直到可能的男性化或女性化性别肯定的适当时间激素疗法。男性化激素治疗通常涉及用于性腺功能减退男性的标准剂量睾酮治疗，以达到典型男性参考范围内的睾酮浓度。

关于 GnRHa 对骨骼影响的人体研究很少。针对跨性别青年的回顾性和小型非对照队列研究表明，即使在 GnRHa 之前，通过双能 X 射线吸收测定法 (DXA) 测量的骨矿物质密度 (BMD) 也可能低于同龄青年。^{( 1-3 )}这很大程度上是由较低的体重指数、较低的维生素 D、钙摄入量不理想和较低的体力活动等因素决定的。^{( 1-3 )} GnRHa 治疗后，BMD 下降，但体脂（包括骨髓脂肪组织）也随之增加。^{( 4 )}性别肯定激素治疗后，BMD Z分数有所增加，但很可能仍低于同龄同龄人。^{( 5, 6 )}但现有研究均采用DXA，其精度较差，与骨折相关性较低。^{( 7 )}此外，GnRHa 导致的体脂增加会影响光子衰减，这可能会人为地低估 BMD。^{( 8 )}研究结果还因使用的参考范围（男性或女性）而异，并且尚不清楚性别确认激素治疗开始的时间是否重要。^{( 9 )}此外，很难推断这些变化是否会影响峰值骨量累积，或者观察到的变化是否与跨性别青少年骨脆性或骨折的长期风险相关。

尽管精确测量人体骨骼微结构或断裂强度的方法具有挑战性，但本月的《骨与矿物质研究杂志》 ( JBMR ) 发表了一项临床相关的转化研究，克服了现有文献中的许多局限性。

Dubois 及其同事模仿通常用于跨性别男孩（出生时被指定为女性）的临床治疗方案^（10），开发了一种小鼠模型，以了解青春期早期抑制对身体成分、骨量和骨强度的影响。

青春期前 4 周大的雌性小鼠接受地加瑞克作为 GnRH 拮抗剂治疗，随后进行安慰剂硅橡胶植入物、从 6 周（青春期早期）开始的睾酮治疗或从 8 周（青春期后期）开始的睾酮治疗。在 16 周时（被认为是成年早期），将结果与未经治疗的雄性和雌性小鼠进行了比较。

地加瑞克治疗显着增加了脂肪量积累（仅限于白色脂肪组织），尽管食物摄入量减少，但脂肪量几乎比任一性别的对照动物高出两倍。^{( 10 )}地加瑞克还可以降低瘦体重和握力。在睾酮替代（无论是早期还是晚期）的两周内，身体成分和骨髓肥胖发生逆转，并且与男性对照组相似。握力恢复到女性水平。

正如预期的那样，与雄性小鼠相比，雌性小鼠股骨干骺端的骨微结构具有较低的小梁骨体积分数（BV/TV）和皮质骨量（通过微计算机断层扫描（μCT）评估）呈性别二态性。地加瑞克将小梁骨量、皮质骨量和股骨断裂强度降低至低于男性和女性对照组的水平。^{( 10 )}

令人欣慰的是，睾酮疗法抵消了地加瑞克的负面影响。BV/TV 增加，主要是由于小梁数量增加和小梁分离减少，因为对小梁厚度的影响微乎其微。^{( 10 )} GnRHa 治疗的持续时间和睾酮给药的时间对小梁 BV/TV 增加的幅度有显着影响。睾酮替代从青春期早期开始，到成年时将 BV/TV 提高到男性对照组的水平；然而，如果在青春期后期开始，BV/TV 只会增加到女性对照组的水平。鉴于小鼠的骨量峰值在 4-6 个月大时达到，^{( 11 )}目前尚不清楚较长时间的睾酮治疗是否可以进一步增加体积骨密度。无论睾酮替代的时间如何，皮质骨量和股骨断裂强度都可以通过睾酮恢复至女性水平。骨长度不受 GnRHa 或睾酮治疗的影响。

尽管小鼠和人类之间存在生理差异，但可以合理地假设，对于跨性别男孩来说，只要生活方式因素针对骨骼健康进行优化（充足的维生素 D、体力活动等），睾酮治疗后骨折风险就不会增加。尽管 GnRHa 治疗的持续时间在临床上通常难以控制（取决于患者何时就医、是否容易获得 GnRHa 以及是否准备好接受激素治疗），但从骨骼和身体成分的角度来看，较早开始睾酮治疗更为有益。当然，在接受既定睾酮治疗的成年跨性别男性中，与顺性别个体相比，似乎骨微结构得到了保留^{（8） ，并且骨折风险没有增加。( 12 )}

这项临床相关的转化研究让接受 GnRHa 治疗和睾酮治疗的跨性别男孩感到放心。需要在人类中进行进一步的研究，特别是在跨性别女孩（出生时被指定为男性）和骨骼结构可能受损的跨性别女性中^（8），并等待跨性别女孩的进一步小鼠模型。